Radio frequency (RF) transmitters are used to transmit RF signals over the air, space, or other transmission medium, to an RF receiver. To compensate for the attenuation that the RF signals experience as they propagate to the receiver, RF transmitters include power amplifiers (PAs) that translate the RF signals to higher power, just before they are transmitted.
The PA is usually the component in the RF transmitter that consumes the most power. For this reason, one of the chief goals normally involved in the design of an RF transmitter is to make the PA operate as efficiently as possible. This goal is particularly important in applications where the transmitter's power supply is a battery, such as in a mobile handset, for example, since the PA's power consumption largely determines how long the RF transmitter is able to operate before its battery must be replaced or recharged.
Designing a PA that operates with high efficiency is complicated. It becomes even more complicated in applications where the PA will be presented with a time varying, i.e., “nonconstant” signal envelope. Many modern wireless communications systems employ nonconstant-envelope modulation schemes, which modulate both the amplitude and phase of the transmitter's RF carrier in order to increase spectral efficiency (the rate that information is conveyed over a given bandwidth). Often the modulated carrier will have a high peak-to-average power ratio (PAPR), so special care must be taken in the design of the PA to avoid clipping the signal peaks of these high PAPR signals. The most straightforward approach to avoiding signal peak clipping is to simply back off the output power of the PA from its peak envelope power, by whatever the PAPR happens to be. Unfortunately, that approach reduces the efficiency of the PA, and even substantially so in circumstances where the PAPR is high. For example, in a Class-A PA topology, which only has a theoretical maximum drain efficiency of 50% to begin with, backing of the output power by 6 dB reduces the PA's maximum possible efficiency to less than 30%.
One commonly used approach that avoids having to back off the output power to avoid signal peak clipping and yet still achieves high efficiency is the polar modulator. FIG. 1 is a drawing showing the salient elements of a polar modulator 100. The polar modulator 100 comprises a PA 102, a dynamic power supply (DPS) 104, and an output matching network 106. As its name suggests, the polar modulator 100 operates in the polar domain, using polar-coordinate amplitude modulation (AM) and phase modulation (PM) components. A major benefit that follows from operating in the polar domain is that the PM component has a constant envelope. The constant envelope affords the ability to operate the PA 102 as a switch, i.e., in “switch mode.” During operation, the DPS 104 receives the amplitude modulation (AM) component (which is representative of the signal envelope of the nonconstant-envelope RF output RF OUT ultimately produced by the polar modulator 100) and produces a DPS power supply voltage VDD(t) that follows the AM. Meanwhile, a constant-envelope, phase-modulated RF carrier carrying the PM is applied to the RF input port of the PA 102. The phase-modulated RF carrier drives the PA 102, switching it between compressed and cut-off states as the DPS voltage VDD(t) produced by the DPS 104 is applied to the PA's 102's power supply port. One important property of a switch-mode PA is that its output RF power depends on the magnitude of its power supply voltage VDD, or, more specifically, on the square of the magnitude of its power supply voltage VDD2. This dependency is exploited in the polar modulator 100 to superimpose the AM contained in the DPS voltage VDD(t) onto the RF output RF OUT as the PA 102 translates the constant-envelope phase-modulated RF carrier to higher RF power.
The output matching network 106 in the polar modulator 100 defines the class of PA (i.e., Class-D, Class-E, etc.) that the polar modulator 100 operates under. In general, the output matching network 106 includes filters that remove unwanted harmonics and that shape the current and voltage waveforms at the output of the PA 102 so they overlap as little as possible, thereby preventing the PA 102 from dissipating wasted power. By operating the PA 102 in switch-mode and carefully designing the output matching network 106, the polar modulator 100 is thus able to achieve very high efficiencies.
Although the polar modulator 100 is able to achieve high efficiencies, its operational capability is constrained by its DPS 104. To maximize efficiency, the polar modulator's DPS 104 is usually implemented using a switch-mode power supply (SMPS). Because the envelope (AM) bandwidth can be very high in modern communications systems, however, the SMPS must be capable of switching at high speeds in order to accurately track the AM. Unfortunately, the power transistors in SMPSs are necessarily large and thus have a limited switching speed capability. Consequently, in circumstances where the DPS 104 is unable to accurately track the AM, significant AM-AM and AM-PM distortion results. Signal envelopes in modern communications applications also tend to have wide voltage dynamic ranges. Designing a DPS that is capable of producing a DPS voltage that covers these wide dynamic voltage ranges can also be difficult, especially when the polar modulator 100 is to be used in situations where the envelope signal bandwidth is high.
Various approaches have been proposed over the years to address the problems that afflict the polar modulator 100 due to its use of the DPS 104. One recently proposed approach that simply eliminates the need for the DPS is the digitally-modulated polar PA, an example of which is described in D. Chowdhury et al., “An Efficient Mixed-Signal 2.4 GHz Polar Power Amplifier in 65-nm CMOS Technology,” IEEE J. Solid-State Circuits, vol. 46, pp. 1796-1809, August 2011. FIG. 2 is a drawing of the digitally-modulated polar PA discussed in that paper. The digitally-modulated polar PA 200 is configured to receive an amplitude code word (ACW) that carries the AM in its encoded bit pattern. The encoded ACW is applied to a decoder 202, which responds by decoding the encoded ACW and switching various of the PAs: PA1, PA2, . . . , PAN into and out of the circuit depending on the logic values of the bits in the decoded ACW. The PAs: PA1, PA2, . . . , PAN are configured so that the output currents of those PAs that are switched into the circuit sum together. In this way, amplitude modulation is accomplished without the need for a DPS. (Note the PM is conveyed to the RF output RF OUT similar to as in the conventional polar modulator 100 described above.)
The digitally-modulated polar PA 200 enjoys the benefit of not requiring a DPS. However, arguably its best attribute is its all-digital capability. Strictly speaking, the PAs: PA1, PA2, . . . , PAN are not “digital” devices. However, from the standpoint that the PAs: PA1, PA2, . . . , PAN operate as switches, which are types of devices that are naturally responsive to digital signals, and given that amplitude modulation is performed under digital control (by enabling and disabling the various PAs: PA1, PA2, . . . , PAN depending on the value of the digital input ACW), the PAs in the digitally-modulated polar PA 200 are, in effect, digital devices. The all-digital control of the PAs: PA1, PA2, . . . , PAN affords the ability to manufacture the digitally-modulated polar PA 200, along with all of its control circuitry, in a single low-cost, all-digital complementary metal-oxide-semiconductor (CMOS) integrated circuit (IC) chip or “system on a chip” (SoC).
Although the digitally-modulated polar PA 200 offers the advantages of: 1) not requiring a DPS; 2) all-digital control; and 3) amenability to being fabricated entirely in CMOS technology, it suffers from one serious problem, which is that it is a highly nonlinear device that produces significant amplitude-to-amplitude modulation (AM-AM) and amplitude-to-phase modulation (AM-PM) distortion. AM-AM distortion and AM-PM distortion occurs in the digitally-modulated polar PA 200 due to the fact that its output impedance varies nonlinearly as a function of the input ACW. Consequently, in order for the digitally-modulated polar PA 200 to have any practical use, some sort of linearization must be applied to correct for its nonlinear behavior.
Various linearization techniques have been proposed to address AM-AM and AM-PM distortion in polar PA architectures. The most widely used approach for conventional polar modulators (like the polar modulator 100 described above) is a technique known as digital predistortion (or “DPD”). In DPD, knowledge of the AM-AM and AM-PM distortion curves of the polar modulator 100 gleaned from measurements, modeling, or simulation data. Predistorted AM and PM data that tracks the inverses of the AM-AM and AM-PM distortion curves is then computed and stored in a look-up table (LUT). During operation, the DSP retrieves the predistorted AM and PM data from the LUT, depending on the input AM and in anticipation of the polar modulator's nonlinear AM-AM and AM-PM response. (Alternatively, rather than storing the predistorted AM and PM data in a LUT, the DSP can be configured to compute the AM-dependent predistorted AM and PM data on-the-fly, based on a mathematical.) The predistorted AM and PM data is then translated downstream, through the AM and PM paths of the polar modulator, so that as the PA in the polar modulator amplifies and modulates the predistorted signals the nonlinearities of the polar modulator are compensated for.
In theory, DPD similar to that used in the conventional polar modulator 100 could also be used to correct for AM-AM and AM-PM distortion in the digitally-modulated polar PA 200. However, there would be serious disadvantages and drawbacks with such an approach. First, not only is each of the PAs: PA2, . . . , PAN that make up the digitally-modulated polar PA 200 a nonlinear device, the collective operation of the PAs: PA1, PA2, . . . , PAN introduces additional nonlinearities that are not easily compensated for using DPD. In other words, the DPD circuitry and methodology needed to linearize the digitally-modulated polar PA 200 would be significantly more complicated than that used to linearize the conventional polar modulator 100. Second, DPD necessarily expands the bandwidths of the AM (ACW) and PM components. Consequently, the expanded bandwidths would require DPD hardware with fast processing speeds, in order to successfully linearize the digitally-modulated polar PA 200. The fast processing speeds would not only make the DPD hardware more difficult to design, it would also increase CV2f losses, which in turn would significantly lower the overall efficiency of the digitally-modulated polar PA 200. The drawback of the need for fast processing speeds is compounded by the fact that the AM and PM components in polar architectures already by their very nature have wide bandwidths. Hence, while in theory DPD might possibly be used to linearize the digitally-modulated polar PA 200, it would not, at least not by itself, be an optimal solution.